1. Field of the Invention
This invention relates to methods and apparatus for scheduling accesses to a shared communications channel.
2. Discussion of the Related Art
Recently, the popularity of wireless local area networks (WLANs) has surged. Now, WLANs provide data connectivity in places such as offices, homes, campuses, and supermarkets. WLANs have even become an integral part of many next-generation communications networks.
A WLAN typically operates without completely centralized control of accesses to the shared wireless channel. Scheduling accesses to a channel without complete centralized control of the accesses is typically referred to as distributed scheduling. In distributed scheduling, the nodes of a local contention neighborhood compete for access to the channel. In particular, the nodes individually determine whether to attempt to access the channel. For that reason, two of the nodes may transmit to the channel during time periods that overlap.
When two transmissions to a channel temporally overlap the transmissions collide. In a wireless channel, a collision often jumbles both transmissions so that a receiver cannot determine the complete content of either transmission. Thus, the colliding transmissions typically have to be resent, and the collision causes an effective reduction in communications bandwidth in a wireless channel.
Though collisions are inherent to distributed scheduling, methods are available for reducing the undesired effects of collisions. Some methods synchronize the temporal slots for transmissions in the channel so that the nodes of a local contention neighborhood agree on the boundaries of time slots for transmissions. For that reason, the nodes of the neighborhood will wait for the beginning of a time slot to attempt a new transmission. This synchronization typically lowers collision rates thereby increasing the bandwidth effectively available for transmissions. Some methods use time periods for competing for channel reservations and separate time periods for transmitting data frames. These methods limit collisions to the time periods reserved for competing for channel reservations. By making the time periods reserved for the transmitting data frames long compared to those reserved for competing for channel reservations these later methods can also reduce effective bandwidth losses associated with collisions.
While the above-described methods help to reduce effective bandwidth losses due to collisions such losses still occur when distributed scheduling is used.